1. Technical Field
The present disclosure relates to commercial aircraft fuel tanks, and more particularly, to controlling fuel tank conditions to lower ignition risks.
2. Related Art
Commercial aircraft fuel tanks are sources of flammability risk. Some commercial aircraft have three main fuel tanks, a wing tank in each wing, plus a center tank, or center fuel tank, between the two wings (often considered part of the wing structure, but physically separated from the outer wing tanks). The wing tanks are exposed to cold outside air when the aircraft climbs to altitude. This cools the fuel in the wing tanks to well below the lower flammability limit (“LFL”) for the fuel. Thus, it is usually thought that only the time spent on the ground and during initial climb out contributes to the cumulative amount of flammability exposure in the wing tanks. The wing tanks therefore typically meet the total exposure limit of 3%, (or other applicable limits established by the relevant regulations) and this has long been accepted by the FAA. The center tank, by contrast, is embedded in the fuselage, often within, or proximate, the environmental control system (“ECS”) bay, so it is not exposed to the cold outside air as much as the wing tanks. Underneath, or proximate, the center tank are typically located air-conditioning packs (A/C packs) that can emit high amounts of heat. This can or could cause the center tank to operate well above LFL for the fuel in the center tank for most of a flight.
Current methods employed by industry to control this flammability risk commonly involve the installation of nitrogen generation equipment that is routed to the fuel tanks, displacing the fuel vapors, and rendering the tanks inert. This method is very costly both for installation of the equipment, and also costly to maintain. They involve many system components, including pre-filters, pressure and temperature regulators, membranes filters, check valves and control circuits and indicators.
In the near term, it is estimated that at least 7,000 of the commercial aircraft operating in the US and Europe will be required to meet new flammability standards imposed by the FAA. These aircraft will all be required to be fitted with equipment that will reduce the flammability exposure to the levels stipulated by the FAA.